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Table 1. Representative Rates at Which Heat and Moisture Are
Given Off by Human Beings in Different States of Activity.
Copyright © 2007, American Society of Heating, Refrigeration and
Air-Conditioning Engineers, It is presented for educational
purposes only. This table may not be copied and/or distributed
electronically or in paper from without permission of ASHRAE
At rest,
over 50% of the sensible heat transfer between the
occupant and the room is radiant based - and yet 99.99% of all
thermostats measure air temperature - go figure.
Visit our
radiant design and performance characteristics page.
Glossary of Terms
Bibliography / resources
For a full overview on building codes and
controlling radiant see the inaugural issue of
BetterBuildings
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Definition of
Radiant Based HVAC Systems
"the Commissioners of the General Board of
Health (1857) advocated as one of the requirements for comfort
that the
walls of a room be at least as high in temperature as the
general temperature of the room, while they included cold walls
or floors amongst the conditions which make for discomfort.”1
National Building Code of Canada v2010:
Section A-5.3.1.2.(1) Use of Thermal Insulation or Mechanical
Systems for Environmental Control states, “In addition to
controlling condensation, interior surface temperatures must be
warm enough to avoid occupant discomfort due to excessive heat
loss by radiation.”
 Many people assume wrongly that
Radiant Based
HVAC Systems are exclusively mechanical or electrical solutions like floor heating.
However, 'radiant' control is ideally
first a building science solution.
This means the building enclosure should be the prime solution
for
controlling the radiant exchange between the occupant and the interior surfaces.
This is achieved with building efficiency.
The higher the performance in the building enclosure the less
temperature difference between the person and the room mass the
more comfortable the occupant. This begins to occur when
loads are reduced with architectural systems to less than 12
Btu/hr/sf.
Mechanical solutions,
like low temperature radiant heating and high temperature
radiant cooling are then applied to compensate for the buildings short comings.
Therefore, when we use the term Radiant Based
HVAC Systems, please understand we are talking about the body’s radiant heat transfer to and from the built environment
(see Table 1.) and controlling it first with architecture then
with mechanical solutions.
As illustrated below, low performing
buildings using traditional heating systems need high fluid
temperatures in heating (and low temperatures in cooling) but
only a high performance building can use low temperatures in
heating and high temperatures in cooling. In fact with high
performing buildings, using radiant cooling and heating,
conductive floors and close tube spacing, fluid and surface
temperatures operate close to the core body temperature.
click to enlarge
Such a small difference between the
fluid temperatures in the pipes and the temperature in
your blood has a major benefit in that it greatly enhance the efficiency of chillers, condensing boilers, solar and heat pumps. So you win when you improve the building and you win
when you use low temperature radiant heating and high
temperature radiant cooling.
To see an example of a high performance building with radiant heating and HRV's for ventilation click here.
See overview of
housing performance categories in North America.
1. Source: Subjective Impressions of Freshness In
Relation To Environmental Conditions by T. Bedford, D.Sc., Ph.D.
and C. G. Warner, Ph.D., B.Sc., of the Industrial Health
Research Board and the London School of Hygiene and Tropical
Medicine, 1939
<http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2199495/>
accessed Oc. 17, 2011
If you wish to discuss this further please visit the
Online Help.
Radiant Based
HVAC – What does it mean to indoor air quality?
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